There have been proposed various apparatus capable of continuously forming a desired functional deposited film on a substrate web in order to obtain a semiconductor device such as a photovoltaic device. Specifically, there are known continuously film-forming apparatus in which a substrate web supplied from a substrate storage chamber is continuously transported through a plurality of reaction chambers to another substrate storage chamber, wherein a given semiconductor film is formed in each of the reaction chambers.
An example of such film-forming apparatus is disclosed in U.S. Pat. No. 4,438,723. This apparatus is for continuously forming, for example, a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer on a substrate web, and it comprises a first reaction chamber for the formation of the p-type semiconductor layer, a second reaction chamber for the formation of the i-type semiconductor layer and a third reaction chamber for the formation of the n-type semiconductor layer, wherein an isolation means is disposed between the first reaction chamber and the second reaction chamber such that the constituent elements of the p-type semiconductor layer are prevented from contaminating into the second reaction chamber, and another isolation means is disposed between the second reaction chamber and the third reaction chamber such that the constituent elements of the n-type semiconductor layer are prevented from contaminating into the second reaction chamber, and wherein the second reaction chamber is maintained at a higher pressure than the first and third reaction chambers upon film formation. According to this continuously film-forming apparatus, it is possible to stack a plurality of semiconductor layers with a different chemical composition on the substrate web. The above patent document does not detail about these isolation means. It is, therefore, understood that each of the isolation means is dedicated simply to isolate the adjacent reaction chambers from each other and the respective adjacent reaction chambers are maintained at a predetermined different inner pressure capable of preventing mutual diffusion of the raw material gases used in the respective reaction chambers. In this case, problems are unavoidably occurred in that since each of the isolation means is provided with a passageway to communicate the adjacent reaction chambers which allows the substrate web to pass therethrough, the raw material gas in one reaction chamber maintained at a relatively high inner pressure is unavoidably allowed to flow through the passageway into the other reaction chamber maintained at a relatively low inner pressure, resulting in not only a change in the inner pressure of the latter reaction chamber but also varying the state of plasma generated in the latter reaction chamber, and because of this, it is difficult to form a desirable deposited film in the latter reaction chamber.
U.S. Pat. No. 4,462,332 discloses a continuous film-forming apparatus provided with gas gate means which is capable of eliminating these problems in the continuously film-forming apparatus described in the foregoing U.S. Pat. No. 4,438,723. This apparatus comprises a plurality of reaction chambers, i.e., a p-type semiconductor layer-forming reaction chamber, an i-type semiconductor layer-forming reaction chamber, and an n-type semiconductor layer-forming chamber, wherein a gas gate means is disposed between each adjacent reaction chambers such that it is situated in close proximity to the i-type semiconductor layer-forming reaction chamber. Each of the gas gate means serves to prevent mutual diffusion of the film-forming raw material gases used in the respective reaction chambers, and it is structured such that gate gas is introduced only from one direction and the gate gas introduced is flown toward the p-type or n-type semiconductor layer-forming reaction chamber. In this continuous film-forming apparatus, each of the gas gate means is provided with a passageway through which a substrate web travels, and a plurality of magnets are provided at the upper passageway wall, wherein the substrate web travels through the passageway while contacting with the upper passageway wall. This results in reducing the size of the passageway. According to this continuously film-forming apparatus, although the foregoing problems in the continuously film-forming apparatus described in U.S. Pat. No. 4,438,723 can be solved to a certain extent, it is still difficult to definitely prevent mutual diffusion of the respective raw material gases used in each adjacent reaction chamber, since the related parameters including the conductance of the gas gate means and the flow rate of the gate gas used are necessary to be accurately controlled to do so. Particularly, in the case of preparing, for example, a pin junction semiconductor device, it is necessary that the p-type and n-type semiconductor layers are made relatively thin and the i-type semiconductor layer is made remarkably thick. Hence, the pin junction semiconductor device is usually prepared in a manner that each of the p-type and n-type semiconductor layers is usually formed, for example, by means of the RF plasma CVD process, and the i-type semiconductor layer is formed by means of the microwave plasma CVD process. In this case, the film deposition pressure in the i-type semiconductor layer-forming reaction chamber is made to be distinguishably lower than that in each of the p-type semiconductor layer-forming reaction chamber and the n-type semiconductor layer-forming reaction chamber, wherein it is required for the dopant-imparting raw material gas used in each of the p-type semiconductor layer-forming reaction chamber and the n-type semiconductor layer-forming reaction chamber not to be contaminated into the i-type semiconductor layer-forming reaction chamber. This requirement is hardly attained by the continuous film-forming apparatus described in U.S. Pat. No. 4,438,723.
In the latter continuous film-forming apparatus, there are adapted the two gas gate means which are disposed at the opposite sides of the i-type semiconductor layer-forming reaction chamber, and each of the gas gate means is designed such that the gate gas is flown toward the p-type semiconductor layer-forming reaction chamber or the n-type semiconductor layer-forming reaction chamber, wherein a back flow is often caused in the gate gas stream, resulting in contaminating the p-type or n-type dopant-imparting raw material gas used in the p-type semiconductor layer-forming reaction chamber or the n-type semiconductor layer-forming reaction chamber into the i-type semiconductor layer-forming reaction chamber. It is extremely difficult to eliminate this problem. Japanese Unexamined Patent Publication No. 30419/1991 proposes a manner to solve this problem in that a gas gate means is disposed at the central position between each adjacent reaction chamber and gate gas is introduced into each of the gas gate means from above and it is exhausted downward. The continuous film-forming apparatus according to this proposal comprises a reaction chamber capable of forming a p-type semiconductor layer by the RF plasma CVD process, a reaction chamber capable of forming an i-type semiconductor layer by the microwave plasma CVD process, and a reaction chamber capable of forming an n-type semiconductor layer by the RF plasma CVD process, wherein one gas gate means is disposed at the central position between the RF plasma CVD p-type semiconductor layer-forming reaction chamber and the microwave plasma CVD i-type semiconductor layer-forming reaction chamber, and the other gas gate means is disposed at the central position between the microwave plasma CVD i-type semiconductor layer-forming reaction chamber and the RF plasma CVD n-type semiconductor layer-forming reaction chamber. Said Japanese document describes that the gas gate means in the continuous film-forming apparatus are operated based on a so-called modified Paschen curve. Even in this case where the gas gate means are thus arranged in the continuous film-forming apparatus, in order to facilitate the effects of the gas gate means so as to definitely prevent mutual diffusion of the respective raw material gases used in each adjacent reaction chambers, it is required to properly reduce the conductance of the slit portion of each of the gas gate means or/and to properly increase the flow rate of the gate gas used. However, there are still remained problems in this case. Firstly, there are the following problems with regard to the conductance of the slit portion of the gas gate means. That is, the conductance of the slit portion of the gas gate means is governed by the cross-sectional shape of the slit portion in the direction of the substrate web to be moved, wherein it is decreased in proportion to the length of the slit portion while it is increased in proportion to the square of the height of the slit portion, and there is a given limit for the width of the slit portion to be narrowed. Other than these, in the case where the conductance of the slit portion is reduced to a certain extent, a problem is apt to occur in that the substrate web vibrates or/and waves upon moving it. In order to move the substrate web without contacting its film deposition face with the wall face of the slit, it is necessary to establish a clearance of about 1 mm or more between the film deposition face of the substrate web and the wall face of the slit opposite the film deposition face of the substrate web. There is, however, a limit for such clearance to be established. In order to decrease the conductance of the slit portion, there is considered a manner of elongating the slit portion. However, there is known a fact that the conductance of the slit is decreased in proportion to the length of the slit. Thus, the slit is necessary to be greatly elongated, and this results in enlarging the scale of the slit. Therefore, such manner is not acceptable in practice. Secondly, there are the following problems with regard to the flow rate of the gate gas used. That is, there is a tendency that the amount of the gate gas flown into the respective reaction chambers is increased as the flow rate of the gate gas is heightened, wherein the film-forming conditions, i.e., the inner pressure, the dilution ratio of the raw material gas, the state of plasma generated, and the like in each reaction chamber are varied, and as a result, there cannot be formed a desirable deposited film in the reaction chamber. In order to prevent occurrence of these problems, there is considered a manner of raising the exhausting efficiency of a vacuuming device used. However, the vacuuming device is necessary to be of a large size to do so. Thus, the continuous film-forming apparatus described in the above Japanese document still has problems to be solved.